Agricultural tractors have traditionally been unsprung. From their earliest beginnings in the late 1800's they have been supported on fixed axles extending from the chassis.
Originally, tractors were used as stationary engines. Located in a fixed position in a field, farmers would gather crops to be threshed and bring them in wagonloads to the tractor and a belt-driven threshing machine. In these early days, the ability to move fast was not important.
Tractors were gradually modified to tow implements such as plows, rakes, harrows, planters, and manure spreaders through agricultural fields. These mobile tractors did not need a great deal of speed since they replaced horses or oxen and needed only enough power to tow implements at horse or ox speed.
As time passed, engineers designed ever larger and stronger implements. To tow these implements, tractors were also made stronger and larger, with ten to fifty times the horsepower of the early tractors.
Eventually, agricultural tractors were capable of towing implements at higher speeds through agricultural fields. To accommodate these greater speeds, manufacturers began to develop front suspensions with springing and shock absorbing capability. These front suspensions were configured to pivot, permitting the front wheels of the tractor to keep a good grip on the ground as the terrain changed. As of today, however, no major manufacturer of tractors sells a commercially accepted agricultural tractor with a sprung rear suspension.
A primary reason that tractors with sprung rear suspensions have not been manufactured is due to the reaction forces that arise when a load is placed on the tractor. Traditional agricultural tractors have large rear wheels, typically on the order of approximately 1 to 2.2 meters in diameter. The large rear wheels apply high force to the ground, especially when a ground-engaging implement is ripping furrows through the ground 2 to 18 inches deep. The ground, in turn, applies an equally high (but in the opposite direction) reaction force on the frame of the tractor, and the reaction force can generate a moment great enough to literally lift the front wheels of a tractor without a rear suspension a meter or more off of the ground.
The existence of a moment large enough to lift the front wheels is best illustrated with reference to FIG. 9, which schematically shows a tractor 700 without a front or rear suspension towing an implement 148. An implement, resultant-force vector 402 is applied to the implement by the ground as the implement is pulled through the ground by the tractor 700. Implement force vector 402 can be broken down into two force vectors 404, 406 that represent the horizontal force (vector 404) acting to drag on the implement during forward motion, and the vertical force (vector 406) that pulls downward on the implement.
The implement is rigidly coupled to the tractor typically through a three-point hitch. The three-point hitch couples the implement to the tractor frame via a lower point A and an upper point B. The implement force vector 402 applies draft forces on the tractor that can be separated into horizontal and vertical forces FAx and FAy acting through the lower link 902 (i.e., at point A) and horizontal and vertical forces FBx and FBy acting through the upper link 904 (i.e., at point B). As one of ordinary skill will appreciate, the relative magnitudes of the component draft forces FAx, FAy, FBx and FBy depend upon the geometry of the three-point pitch.
Other forces acting on the tractor 700 include weight (depicted in the drawing as mg), which acts on the center of gravity CG. In response to the weight, the ground applies forces Ff and Fr to the tractor through the front and rear axles, respectively.
There are torques shown in FIG. 9 as well. Drive torque TD is the torque applied by the engine (shown in FIG. 1) to the axle (also not shown) to drive the rear wheels. When the tractor is being driven forward, the drive torque is clockwise. The rear wheels, as they are being driven, apply a force on the ground, and the ground, in turn, applies an equal and opposite traction force FTr on the wheels that is applied to the tractor frame. The traction force of course is responsible for forward movement of the tractor.
Drive torque TD also generates a reaction torque (that is, traction torque TTr) that acts on the frame of the tractor. The traction torque is proportional to the traction force FTr and is counterclockwise.
The forces and torques generate moments about a point on the tractor that tend to rotate the tractor about that point. For convenience, the point will be called the center of pitch Cp. Its location depends upon a number of factors one of ordinary skill will appreciate. While the forces and torques may generate moments that cancel each other out to some extent, the net effect of all of the moments is to generate a counterclockwise moment Mp about the center of pitch when the implement force vector 402 increases. The implement force vector increases when the implement 148 hits a stone, compacted soil, or some other such condition. As previously mentioned, the increased implement force vector can be large enough to cause a moment Mp about the center of pitch that is itself large enough to lift the front tires and increase the load on the rear tires.
If the rear wheels were suspended on the frame rather than being fixed, the moment Mp will not at first lift the front wheels, but it will tend to cause the rear suspension to squat. Such squatting can be disconcerting to the operator and can also wreak havoc on implement depth-control systems, which typically require a constant relationship between the tractor-frame and implement-frame heights.
One of ordinary skill will appreciate that some suspension configurations will cause the tractor to rotate clockwise (rather than counterclockwise, as has been described) when the tractor is subjected to increased loads. However, for the purposes of this discussion, we will consider the more intuitive case where the tractor rotates counterclockwise in response to increased loads. Nevertheless, the basic principles (and the problems with conventional systems) described herein are the same. Moreover, the principle of operation of the preferred embodiments (which will be described below) is the same regardless of whether the suspension tends to squat or sit up.
The suspension arrangement of the present invention generates a reaction torque on the vehicle to reduce the moment Mp about the center of pitch. In other words, when the tractor pulls harder on its implement, the suspension in accordance with the present invention generates an increased counteracting, or reaction, force that matches or is proportional to the increased, horizontal force vector 404. Similarly, when the tractor pulls more gently on its implement, the suspension in accordance with the present invention generates a decreased force that matches the decreased horizontal force vector 404.
The applicant, in his co-pending patent application U.S. patent application Ser. No. 10/911,888, described a tractor that would solve many of these problems.
One problem that was not addressed by the tractor of that application was the rigidity of the rear suspension. The tractor shown in the prior application had suspension arms extending longitudinally toward the rear of the vehicle, but these suspension arms would flex unduly when the extreme torques generated by the engine were applied to the rear wheels.
The rear suspensions, as best shown in FIGS. 2-3 of that application, included a trailing link suspension arm pivotally coupled to the chassis of the tractor at its front and free to pivot up and down at its rear end. The rear of the suspension arms included a planetary gear system from which a laterally extending axle protrudes. The tractor's rear wheels are fixed to the ends of these axles. The axles extend a significant distance laterally away from the planetary gearbox, and the wheels are large diameter tractor wheels that extend a significant distance vertically from the axle to the ground. The distance between the tire/ground contact point and the pivot point of the suspension arm with respect to the chassis is typically 1-2 meters vertically, 0.5 to 2 meters horizontally and 1-1.5 meters longitudinally. This arrangement, which is the same as shown herein, applies considerable bending and twisting stresses on the suspension arm, particularly when a ground-engaging implement is fixed to the tractor.
What is needed, therefore, is a system that resists the twisting and bending forces applied to the suspension arm. What is also needed is a system that resists lateral inward and outward movement of the free end of the suspension arm. What is also needed is a system that will resist lateral deflection of the suspension arm and suspension arm movement of the free rear end of the suspension arm yet will also permit the suspension arm of the tractor to pivot up and down with respect to the chassis. It is an object of this invention to provide a tractor and tractor suspension that have these benefits.